You want to support the durock. If it doesn't break there is a good chance it will bow. Just run a couple of 2x4's length wise and brace them. It's going to weigh a lot more than 800 lbs because you will be adding a lot of water.

Gudday
I secound that remark, I have been down this path before,make yourself a platform so you step up to throw your mix in, you shoulders and back will appreciate it at the end of the day.
Regards dave

Your slab is going to come out just fine! You have joists below that support your durarock and it is only 3 1/2" thick--so it is not going to give any edge pressure on your 2x4's.

I just did a little slab like this yesterday for under a unit bath. That is a KD bath enclosure that will be installed tomorrow. I calculated your volume and mine was almost exactly the same. I mixed the concrete with a 1/2" mixer drill and paddle. Dumped the dry mix into my 60 liter rubber bucket and whipped the stuff up. My mixture calls for 25KG dry mix concrete and 3.8 liters water. I found that a little stiff for the drill so I added another .75 liters.

If you measure your water accurately, you can make repeated batches and the concrete will always be consistant. Don't know how much portland cement is in your dry mix concrete--so cannot give you any recommendations. The bag should have the recipe listed. follow the recommendations and it will be fine.

with a mixer drill used by plasterers on small jobs, I mixed up my cement in about an hour. 15- 25kg bags. (825lbs) , Yours - 10 - 80#bags (800 lbs). Better if the cement is a little stiff, then use a float trowel---you can make it with a piece of wood--(reacts like a magfloat) and agitate the concrete so that it surrounds the rebar consistantly and fills the corners of the form. Once you have all the concrete in the form, you can use a (maybe 6' straightedge-and agitate it back and forth over the surface--using your form as a guide. It does not have to be glass smooth just the same height.

The next day, you can still knock down high spots with a piece of cement block--just rub it across the surface if you have any little humps.

Just try to get it as level as possible before it gets too hard. Don't spend a lot of time trying to get things glass smooth. If you were a concrete finishing professional, you would know how many times to trowel the surface--best to just get it flat then leave it alone!

Good luck on your pour, I see that you did not have anything to make 90 degree bends to the rebar! so what, it will work fine with whatever you stick into the form as long as it is metal.

[QUOTE=mikku;153781 Dumped the dry mix into my 60 liter rubber bucket and whipped the stuff up. My mixture calls for 25KG dry mix concrete and 3.8 liters water. I found that a little stiff for the drill so I added another .75 liters.

[/QUOTE]

By adding that much more water your concrete will be weaker. If you use a superplasticizer you can get it more fluid without having to add that extra water.

I added the additional water just to get it to mix. The resulting concrete was not sloppy, it stood up all by itself. Very little slump.. The description might sound like I threw in a bunch of water in until I got pea soup, it wasn't. The first batch I measured exact amounts as listed and it did not blend at all--then added 250ml additional water until I got the desired mix. After that all additional batches had the exact water ratio. The dry mix cement that was brought to site in bags had very high content portland cement. Local name is "pacific umi cement".

I will have to look into the superplasticizer you talking about..but need to know a kangi equivalent to even search.

The point I was attempting to make was--you don't need a bulky mixer to do concrete successfully. the mixer I use is a Toshiba-two speed, no chucks- just threaded shaft to accept different mixing heads. Works great, easy to use, lightweight, can throw it into the back of the truck to make cement anytime--big orders call the ready mix company--less than truck sizes--use a mixing drill. Also mixed 2- 25kg bags at a time, took a couple of minutes each batch.

Ok, you did post that you added .75 litres extra. That's around 20% more than recommended- too much IMO. If you only added .25 L you should probably be ok. Try searching for "excess water in concrete" and "super plasticizer" to find out the importance of not adding more water than the recipe states.

Thank you all for the good advice from all over the world. It was an absolutely perfect memorial day today. My neighbor Ernie came over this morning and helped me. We built a platform for the mixer. Sorry, I didn't take a picture. The pour went well. Ernie was in charge of filling up the voids, so if there are voids it is his fault.

It wound up taking 8 bags.

We thought that the first few bags were too dry, and perhaps the last couple a bit too wet, so perhaps they will cancel each other out. Is that wishful thinking?

You can see that there was not too much spillage at all. The signature has gone a long way to keeping the wife's approval of the project.

Everything seemed pretty level. The back right corner wound up a touch low, but for my first effort at this, I think it all in all pretty good. That spot won't be under the oven anyway.

I kept misting the slab all day, as it was hot and sunny. I'm going to be out all day tomorrow, so I hosed the slab down, and bungeed this tarp on top.

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